The retention of time in standalone digital radiocommunication equipment is usually handled by a quartz oscillator, the output of which, generally after a divider, regularly increments a counter. The time can be deduced from this counter. The precision of the time supplied should observe criteria linked in particular to the bit rate of the radiocommunications and to the inactivity time between communications. Also, the pulses produced by the oscillator after division may be used to drive sequencers managing the state of the radio equipment, in particular when the equipment is shut down or when the latter is on standby, or even provoke processor interrupts. These pulses have to observe phase jitter constraints.
Producing such a device presupposes finding the most satisfactory trade-off between a number of conflicting objectives, namely:                research and fabrication costs that are as low as possible,        operation at a low voltage which should observe a low consumption to enable the time to be retained on an integrated energy source (cell battery or accumulator battery) over a duration that may exceed a year in certain cases,        maintenance of time precision in the short, medium and sometimes long term that is sufficient to maintain the operational functionalities of the equipment, notably its capacity to communicate after a period of inactivity. This last constraint should take into account the variation of the operating conditions of the device: power supply voltage which decreases over time, temperature which may vary between defined limits, speed of variation of this temperature.        
A first solution proposed by the prior art relates to the production of a precise and low-consumption clock based on the use of a quartz oscillator with moderate frequency, below 10 MHz, operating at low power, followed by a divider. The relatively high variations of such an oscillator as a function of temperature are compensated by a learning-based calibration with a sufficient number of temperatures and a storage of parameters in a non-volatile memory. The correction is handled by a processor circuit which is periodically placed in service to read the non-volatile memory, to generate an analog voltage which is applied to a varicap diode which then adjusts the frequency of the oscillator as a function of the stored value. The variations of the oscillator as a function of temperature are sufficiently low for a fairly slow correction rate (for example every 10 seconds) to be sufficient to tolerate fairly rapid temperature variations, for example 3° C./min. The design of such an oscillator is difficult and the production cost is high. Also, the consumption still remains relatively high because of the high quartz frequency and proves difficult to bring below 400 microamperes. One of the drawbacks of this prior art is that it uses a quartz oscillator operating at low power, and requires a specific, difficult and costly design, with a relatively high consumption.
In order to reduce the consumption, one solution consists in reducing the frequency of the oscillator to a value corresponding to that generally used in watches, that is to say 32 768 KHz. The natural stability of the oscillator is then significantly less good and its compensation by calibration as a function of temperature is more difficult. The desired precision is not generally achieved, for example 2 ppm in a wide temperature range that may vary between −40° C. and +85° C.
Another solution consists in employing two oscillators:                a high frequency oscillator, above 10 MHz, with high precision, temperature-compensated, with limited aging, but relatively high consumption; such a component is known by the abbreviation TCXO (Temperature-Compensated Crystal Oscillator); its cost is relatively low. This oscillator is the main clock of the equipment and it is used in operation to pace the processors; it is not powered when the equipment is on standby or shut down.        a low-frequency oscillator, generally of the order of 32 768 KHz, imprecise, not temperature-compensated, with higher aging, but very low consumption and very low cost; this oscillator is kept operating on standby and sometimes when shut down.        
The drawbacks created by the use of the low-frequency oscillator are generally compensated by one or more of the following solutions:                calibration of the low-frequency oscillator in the factory as a function of temperature and storage of the error in a non-volatile memory (error measured during calibration); periodic application of the correction on standby or when shut down,        automatic calibration of the low-frequency oscillator relative to the high-frequency oscillator when the equipment is in operation and storage of the error in a non-volatile memory,        automatic calibration of the low-frequency oscillator during communications with a base station and storage of the error in a non-volatile memory,        inclusion of typical parameters of the low-frequency oscillator: probable aging, variation as a function of power supply voltage.        
In the case where two oscillators are used, the solution is not generally designed for generation of a permanent precise compensated clock, independent of the rest of the equipment. In the case where the clock is physically generated, the switching between the two oscillators produces an uncertainty as to its phase, an uncertainty which may be incompatible with the desired time precision and the specified jitter.
European patent application EP 1585223 describes a method and a system for determining a calibration factor between a fast clock signal and high accuracy, and a slower clock signal. EP 1585223 proposes to apply the clock signal slower (20 to 100 kHz) with a variable order divider, which rank is stable and is adjusted periodically even if it does not work, after rapid counting the clock signal for a period of the slower clock signal. This process does not achieve an important point, because of the low count time and the variation in frequency between each division rank. It is referred to a precision better than 5%.
The solutions proposed by the prior art are therefore either too costly, too energy-intensive for a system operating on a cell battery or that has a low power or energy autonomy, or insufficiently precise in frequency or phase.